1. Technical Field
This invention relates generally to a filtration system for an exhaust gas recirculation system, and more particularly to such a filtration system having separate electrochemical and particulate filter stages.
2. Background Art
Increasingly stringent emission regulations call for a major reduction in NOx emissions. Engine manufacturers have developed systems for exhaust gas recirculation (EGR) to achieve lower NOx standards. Using EGR in diesel engines has real advantages at low engine load conditions where high air/fuel (A/F) ratios are dominant. At high engine load conditions, A/F ratios are greatly reduced and may reach values as low as 20:1. Low A/F ratios contribute to excessive smoke formation. Recirculating exhaust gas with high soot content, whether at high or low A/F conditions, has adverse consequences on engine durability, lubrication oil quality, and on the service life of exhaust gas recirculation system components.
Exhaust gases carry a number of chemical constituents that result from the combustion process. These constituents normally include; unburned hydrocarbons, carbon monoxide, carbon dioxide, and nitrogen oxides, along with other gases. Most, generally about 90%, of nitrogen oxides emitted from diesel combustion are in the form of nitric oxide (NO).
In high pressure loop exhaust gas recirculation systems, i.e., EGR systems for turbocharged engines with the exhaust gas recirculated directly between the exhaust and intake manifolds, all components of the EGR system, such as heat exchangers and control valves, are subjected to exhaust gas containing all of the untreated products of combustion. The untreated exhaust gas shortens the service life of the EGR system, necessitating frequent cleaning of the heat exchanger surfaces to prevent fouling, or even closure of the gas flow path through the exchanger, as well as frequent cleaning of the EGR flow control valve to prevent fouling which would avoid proper operation of the valve.
Previous attempts at reducing undesirable products of combustion from exhaust gas in turbocharged engines have primarily been directed to treating the exhaust gas downstream of the turbocharger, with the treated recirculated exhaust gas being introduced upstream of the compressor stage of the turbocharger. For example, PCT International Application No. WO 99/09307, published Feb. 25, 1999, describes a low pressure loop exhaust gas recirculation system incorporating a catalyst and a particulate trap disposed in the undivided exhaust stream downstream of the turbocharger.
Other emission reduction systems have also been directed at reducing NOx and hydrocarbon emissions from the overall exhaust gas discharged from an engine. An exhaust gas treatment system comprising a storage device for collecting NOx hydrocarbon, or particulate emissions, and a plasma reactor for destroying the collected emissions, is described in U.S. Pat. No. 5,746,984, issued May 5, 1998, to John W. Hoard and entitled EXHAUST SYSTEM WITH EMISSIONS STORAGE DEVICE AND PLASMA REACTOR. Neither of the above described exhaust gas treatment systems have effectively addressed the problem of specifically treating exhaust gas recirculated through a high pressure loop EGR system.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a filtration system for a high pressure loop exhaust gas recirculation system in which the filtration system reduces particulate matter and other deleterious products of combustion circulated through components of the EGR system. It is also desirable to have such a filtration system which not only traps particulate matter from the high pressure circulated gas stream, but also has the means for removing the trapped particulate matter from the filter. Furthermore, it is also desirable to have an EGR filtration system that interacts with the gas molecules in the exhaust gas stream, and thereby creates free radicals in the recirculated exhaust gas stream that have a longer life and thereby enhance combustion of fuel mixed with the recirculated exhaust gas.
In accordance with one aspect of the present invention, an exhaust gas recirculation filtration system for an internal combustion engine having an intake manifold and exhaust manifold in direct fluid communication with at least one combustion chamber of the engine includes a non-thermal plasma generator having an intake port in direct fluid communication with the exhaust manifold of the engine, and an exhaust portion spaced from the intake port. The filtration system also includes a particulate filter having an intake port in direct fluid communication with the exhaust port of the non-thermal plasma generator and an exhaust port spaced from the intake portion. The exhaust port of the particulate filter is in controlled fluid communication with the intake manifold of the engine.
Other features of the exhaust gas recirculation filtration system embodying the present invention include the filtration system being one component of an exhaust gas recirculation system having a flow control valve interposed between the exhaust port of the particulate filter of the filtration system and the intake manifold of the engine. Other features include the exhaust gas recirculation system having a heat exchanger interposed between the exhaust port of the particulate filter of the filtration system and the flow control valve.
Still other features of the exhaust gas recirculation filtration system embodying the present invention include the non-thermal plasma generator being a pulsed corona discharge device providing high voltage pulses in a range of from about 30 kv to about 40 kv, each having a width of from about 5 ns to about 10 ns at a frequency of about 100 Hz. Still additional features include the internal combustion engine being a turbocharged engine having a compressor stage disposed between an air intake duct and the intake manifold and mechanically driven by a turbine stage disposed between the exhaust manifold and an exhaust duct of the engine. In this embodiment, the exhaust gas recirculation system embodying the present invention is disposed between the exhaust manifold and the intake manifold of the engine, thus forming a high pressure loop exhaust gas recirculation system for the engine.